In machine tools, there are cases in which a workpiece is processed using multiple shafts in synchronism. As for such cases, as a machining process by synchronizing two shafts, namely, the first and second shafts, with a fixed synchronous ratio (sync ratio), examples of which are as follows.
As a first example, a thread cutting process is known. FIG. 1 is a schematic diagram of a machining system for performing a thread cutting process. A thread cutting system 1000 performs a machining process by synchronizing a master motor 1002 for turning a rod-shaped workpiece 1001 to be processed with a slave motor 1004 for moving a cutter 1003 as a tool. By moving the cutter 1003 in synchronization with the revolution of the rod-shaped workpiece 1001 at a predetermined ratio, a male thread is formed on the rod-shaped workpiece 1001. Alternatively, the rod-shaped workpiece 1001 may be moved while the cutter 1003 is fixed.
As a second example, a tapping (rigid tap) process is known. FIG. 2 is a schematic diagram of a tapping system. A tapping system 1010 machines a workpiece 1011 for processing by synchronizing a slave motor 1014 for turning a tapper 1013 as a tool with a master motor 1012 for moving the tapper 1013 as a tool. By moving the tapper 1013 in synchronization with the revolution of the tapper 1013 at a predetermined ratio, a female thread is formed inside a bore of the workpiece 1011. Alternatively, the workpiece 1011 may be moved while the tapper 1013 is fixed.
As a third example, a gear generating process is known. FIG. 3 is a schematic diagram of a gear generating system. In a gear generating system 1020, a workpiece gear 1021 for processing is fixed to the workpiece shaft, namely C-shaft, which is turned by a slave motor 1024. On the other hand, a grinder 1023 as a tool is fixed to the tool shaft, namely B-shaft, which is turned by a master motor 1022. Machining is performed by synchronizing the revolution of the tool shaft (master) with that of the workpiece shaft (slave) at a predetermined ratio (=number of teeth of the cutter/number of teeth of the gear, which will be referred to herein below referred to as “sync ratio”).
Generally, in the machine control as above, the following two methods are used.
1) Feedback Tracking Method
Feedback tracking method is a method whereby the feedback of the master shaft is multiplied by the synchronous ratio so as to be used as a command to the slave shaft (for example, Publication of Japanese Examined Patent Application (Kokoku) No. 59-35729).
FIG. 4 is a schematic diagram of a machining system using a feedback tracking method. A servo controller 1040 using a feedback tracking method includes a first servo control unit 1043 for controlling a master motor 1033 that turns a tool 1031 about a master shaft and a second servo control unit 1041 for controlling a slave motor 1034 that turns a workpiece 1032, and multiplies the feedback of the master shaft by a synchronous ratio (gear ratio) at a multiplier 1042 so as to produce a command to the slave shaft.
2) Command Distribution Method
Command distribution method is a method whereby a command to the master shaft is multiplied by a synchronous ratio to produce a command to the slave shaft (e.g., Japanese Patent Application Laid-open No. 2003-200332 (JP2003-200332A)).
FIG. 5 is a schematic diagram showing a machining system using a command distribution method. A servo controller 1050 using a command distribution method includes a first servo control unit 1053 for controlling a master motor 1033 that turns a tool 1031 about a master shaft and a second servo control unit 1051 for controlling a slave motor 1034 that turns a workpiece 1032, and multiplies the command to the master shaft by a synchronous ratio (gear ratio) at a multiplier 1052 so as to produce a command to the slave shaft.
The above 1) has the advantage that use of the feedback of the master shaft as a command to the slave shaft makes it possible to suppress synchronous error even when the speed of the master shaft is changed. However, if there is a vibration depending on the rigidity of the tool, workpiece and/or the mechanical part for driving these, the vibration may be amplified by the loop as follows to make the system unstable, and therefore this method has the following disadvantage:—                vibration→master (tool)→master shaft feedback→slave shaft command→slave shaft control→slave (workpiece)→master (tool).        
On the other hand, since in the above 2) the command to the master shaft is used to command the slave shaft, the aforementioned loop is not formed, and therefore the above 2) has the advantage that no vibration amplification will occur and is excellent in stability. However, this method has the disadvantage that synchronous error occurs with respect to the speed change of the master shaft.
Conventionally, the above two methods have been used selectively depending on the machining condition and mechanical condition. For example, in a machine that uses a speed reducer for the master shaft, the servo stiffness of the master shaft cannot be made high. Accordingly, if a strong machining disturbance acts, speed variation of the master shaft arises. In such a case, the above control method 1) is used.
On the other hand, when vibrations occur depending on a load applied during machining, the rigidities of the tool, workpiece and the mechanical part for driving these, the above control method 2) is adopted in order to avoid amplification of vibrations resulting from the aforementioned loop.
In this way, in the prior art, there has been the problem that synchronous error cannot be sufficiently lowered in a machine in which the master shaft is low in servo stiffness, hence speed change may occur due to machining disturbance, and therefore vibrations depending on the mechanical rigidity occur.
In this case, in the conventional method, on the basis of the above control method 2) the synchronous error between the master and slave shafts is used to correct one of the shafts.
For example, there has been a known method to correct the positional deviation of the slave shaft based on the synchronous error between the master shaft and the slave shaft to be synchronized with the master shaft and learning control (e.g., Japanese Patent No. 4361071 (JP4361071B)). According to this conventional technology, the positional deviation of the slave shaft is corrected using synchronous error and learning control so as to reduce periodical synchronous error, on the basis of the above control method 2). This method can reduce synchronous error deriving from periodical deviation by learning control, but has the problem that aperiodic synchronous error cannot be suppressed. This method discloses a filtering means for band limitation. This is valid for positional correction against periodic synchronous error below the predetermined band, but cannot make any positional correction for aperiodic synchronous error even if the error is below the predetermined band, and worse still, may amplify the error.
In general, vibrations that occur are based on the rigidities of the tool, and therefore the workpiece and the mechanical part for driving these are of high frequencies. For example, the high frequency herein is equal to or higher than 100 Hz, which is in the range difficult to handle by servo control. On the other hand, the vibrations due to load disturbance during machining and the frequency components synchronized with revolutions of the tool and the workpiece are of low frequencies. This low frequency is less than, for example 30 Hz, which is in the servo controllable range.
The object of the present invention is to provide a servo controller for performing control by synchronizing two shafts, which can reduce synchronous error arising between the two shafts.